Very long optical fiber transmission paths, such as those employed in undersea or terrestrial lightwave transmission systems and which employ optical amplifier repeaters, are subject to decreased performance due to a host of impairments that accumulate along the length of the optical transmission path. The source of these impairments includes amplified spontaneous emission (ASE) optical noise generated in the Erbium-Doped Fiber-Amplifiers (EDFAs), nonlinear effects caused by dependence of the single-mode fiber's index on the intensity of the light propagating through it, and chromatic dispersion, which causes different optical frequencies to travel at different group velocities. Typically it is advantageous to operate long-haul transmission systems at high data rates per channel. For example, useful data rates include multiples of the Synchronous Digital Hierarchy (SDH) standard of 2.5 Gb/s. As the bit rates rise into the gigabit per second range it becomes more critical to understand the accumulated dispersion in the transmission medium.
Chromatic dispersion can directly effect the quality of the transmission by distorting the waveform in a manner that is often referred to as a dispersion penalty (for example, see P. S. Henry et al., "Introduction to Lightwave Systems," Chapter 21 in Optical Fiber Telecommunications II Academic Press 1988). Chromatic dispersion can also indirectly effect the quality of the transmission through the fiber's nonlinear index of refraction. For long-haul systems the nonlinear refractive index can couple the data signal with optical noise or with different signal channels in a wavelength division multiplexed system. Chromatic dispersion can reduce the propagation distance over which closely spaced wavelengths overlap (known as phase matching). Accordingly, chromatic dispersion can reduce the amount of interaction through the nonlinear index in the fiber. Therefore, it is important to understand the fiber cable's dispersion characteristics to satisfactorily operate an optical transmission system.
Accurate systems for measuring the chromatic dispersion in single-mode fibers are commercially available. More recently, techniques have become available that can accurately measure the dispersion characteristics of concatenated sections of single-mode fiber and optical amplifiers, such as described by Horiuchi in "Chromatic Dispersion Measurements of 4564 km Optical Amplifier Repeater System," Electronics Letters, Vol. 29, No. 1 1993 p4-6. Typically, dispersion is measured by intensity modulating a tunable optical source, and measuring the delay difference versus the transmitted wavelength. Accurate delay measurements in the picosecond range are achieved by comparing the transmitted and received timing information, usually in the form of a phase detection. Unfortunately, most of the existing techniques require the two ends of the optical cable to be located in close proximity to each other, which is not always possible before a cable has been installed and which is virtually impossible after it has been installed. Moreover, international transoceanic cables systems are often developed by more than one supplier and typically various cable sections are connected only after the different supplier's sections have been installed. Thus, the completed cable is often not available to properly measure its chromatic dispersion characteristics. Therefore, it is important to be able to measure the chromatic dispersion of installed cables, where of course, the cable ends are in different locations. It is also important to be able to determine the chromatic dispersion of installed cables because the dispersion characteristics of cables can change due to a variety of effects such as temperature and pressure and therefore, even if the cable could be accurately measured in the factory, the value of dispersion could be different in the field.